1. Field of the Invention
The present invention relates to an echo cancellation processing system applied to a full duplex telephony system.
2. Description of the Related Art
Various services using the Internet start being provided along with the widespread use of the Internet, and a so-called computer telephony system using a technique of VoIP (Voice over Internet Protocol) or the like via the Internet is becoming widespread. Hereinafter, a conventional full duplex telephony system using a VoIP application via the Internet will be described.
FIG. 9 schematically shows a fill duplex telephony system. In FIG. 9, references numerals 510 and 520 denote speakers. For convenience, the case will be described where the speaker 520 utters a speech, which is transmitted to the speaker (communication partner) 510. Reference numerals 511 and 521 denote microphones, 512 and 522 denote loudspeakers, 513 and 523 denote VoIP applications, 514 and 524 denote terminal apparatuses, and 530 denotes the Internet. A communication interface, and other devices are not shown for convenience of description.
When the speaker 520 inputs a speech to the microphone 521, the VoIP application 523 receives the speech and conducts required processing such as sampling, whereby the speech is sent from the terminal apparatus 524 to the Internet 530 as packet data. Each packet data follows routing on the Internet to reach the terminal apparatus 514. Then, each packet data is assembled in a correct order, and subjected to required processing in the VoIP application 513. Thereafter, the packet data is outputted from the loudspeaker 512 as a speech signal.
Herein, it is conventionally known that a phenomenon called an echo occurs. While the speech outputted from the loudspeaker 512 reaches the speaker 510, it may be inputted to the microphone 511 as a speech due to wraparound. In this case, the speech inputted to the microphone 511 is transmitted via the VoIP application 513 of the terminal apparatus 514, the Internet 530 and the VoIP application 523 of the terminal apparatus 524, and outputted as a speech containing an echo from the loudspeaker 522, thereby forming a kind of loop. A transmission delay is caused in a path (echo path) forming a loop that causes the echo. More specifically, the speaker 520 hears his/her speech, which has been inputted to the microphone 521, from the loudspeaker 522 after a slight delay. It is known that in the case where such an echo is caused, the speaker has difficulty in hearing a speech of a communication partner, as well as in speaking. Furthermore, in the case where the level of an echo is very high, and the echo diffuses without being attenuated, a telephone conversation becomes disabled due to a phenomenon called howling.
In order to solve the above-mentioned problem, an echo canceller is conventionally used. FIG. 10 schematically shows an echo cancellation processing system using an echo canceller in the prior art. In FIG. 10, the terminal apparatus 514 on the speaker 510 side has an echo canceller 515. The echo canceller 515 captures a signal to be outputted through the loudspeaker 512, and removes the captured signal component from a signal captured through the microphone 511, thereby canceling a speech signal inputted to the microphone 511 due to wraparound.
The echo canceller 515 includes an adaptive filter, a coefficient updating part, and a subtracter. The adaptive filter generates an estimated signal in accordance with the amount of an echo speech signal that is outputted from the loudspeaker 512 and wraps around to the microphone 511. The subtracter subtracts an estimated signal component generated by the adaptive filter from an input speech signal obtained through the microphone 511. If a wraparound speech signal component contained in the input speech signal is exactly subtracted in the subtracter, an echo can be completely deleted.
Herein, the prior art is predicated under the following conditions: the influence of a relative positional relationship between the loudspeaker 512 and the microphone 511 and a surrounding environment upon an echo is known. Furthermore, the delay amount of a speech signal wrapping around to the microphone 511 from the loudspeaker 512 is considered to be constant, and the coefficient of the adaptive filter is set so as to substantially correspond to the delay amount and surrounding environment determined based on the known information. However, the delay amount and surrounding environment are not always invariable, and an initial setting is not necessarily optimum, so that the coefficient updating part is provided. The coefficient updating part feeds back the result of echo cancellation processing, and updates the coefficient of the adaptive filter at a predetermined timing.
However, the echo cancellation processing system using the conventional echo canceller has the following problems.
First, in the conventional echo canceller, when an environment around a speaker is varied largely, echo cancellation processing cannot be adjusted immediately in accordance with the variation.
The conventional echo canceller is effective under the following conditions: a relative positional relationship between the loudspeaker 512 and the microphone 511 is constant, a variation in environment at a place where a speaker system is disposed is small enough, and a change in delay amount of a speech signal that wraps around to the microphone 511 from the loudspeaker 512 is small. Therefore, in the case where these preconditions are not satisfied, and a variation in environment is large (e.g., in the case where the conditions of sound reflection from a wall and a ceiling are varied, in the case where the relative positional relationship between the loudspeaker and the microphone due to the movement of the loudspeaker or the microphone is changed largely, etc.), a change in delay amount of a speech signal that wraps around to the microphone from the loudspeaker is large, and such a large change cannot be immediately cancelled by updating the coefficient of the adaptive filter in the coefficient updating part of the conventional echo cancellation processing system.
Second, due to the presence of a noise source, an echo cancellation effect is likely to decrease.
In order to enhance the performance of an echo canceller, it is important to capture a speech output signal of the loudspeaker to be inputted to the microphone with good sensitivity. However, the conventional echo canceller is likely to be influenced by a noise source such as a speaker's speech and a background sound in an environment. In the case where there is such a noise source, an estimation precision of a speech output signal of the loudspeaker to be inputted to the microphone is lowered, and the parameter updating amount of the adaptive filter in the coefficient updating part does not become appropriate, which decreases the performance of echo cancellation processing.
Third, according to the conventional echo cancellation processing system, in the case where echo cancellation processing is not sufficient, the quality of telephone conversation decreases.
According to the conventional echo cancellation processing system, even in the case where echo cancellation processing is not sufficient, echo cancellation processing is continued, and a residual signal after the echo cancellation processing is also sent to a system on the communication partner side. Thus, a residual signal is always sent to a speaker system as a background noise, which degrades the quality of telephone conversation.